Many valve spools of hydraulic control valves have a plurality of metering slots or notches formed thereon interconnecting a groove and an adjacent land. Since the action of opening or closing a port with a simple spool shoulder is relatively abrupt, the slots provide a slow metering effect and thus a desired gradual or smooth action of the fluid motor or jack controlled by the valve. Two of such slot configurations are disclosed in the U.S. Pat. Nos. 2,971,536 and 3,198,212 assigned to the assignee of the present application. The construction disclosed in the first mentioned patent is particularly suited for metering out while the construction of the last mentioned patent is best suited for metering in. The term "metering in" is employed in describing a valve wherein the metering is accomplished by fluid flow through the slots in a direction generally inward with respect to the valve spool whereas "metering out" is used to describe the condition where metering is accomplished by flow radially outwardly of the valve spool through the metering slots.
One of the difficulties encountered with the construction of the U.S. Pat. No. 2,971,536, is that of sizing the throttling notches to maintain sufficient cross sectional area for adequate strength in the area of the notches while providing acceptable fluid flow rates therethrough without creating an excessive pressure drop or a reactionary force from the jet flow of the fluid. In many cases, particularly on relatively small diameter spools, to obtain the desired fluid flow characteristics through the throttling notches, the notches are formed relatively deep in the spool creating a weakened area subject to possible breakage.
Conversely, reducing the size of the throttling notches to provide adequate strength reduces the cross sectional area of the fluid flow path in the face of the land so that the opening in the face acts as an orifice restricting fluid flow therethrough and creates a pressure drop between the high pressure fluid in the annular groove and the fluid in the throttling notches. This creates a jet flow of fluid through the opening in the face of the land such that the high velocity fluid impinges against the back of the slot creating a reactionary force on the spool. This requires either heavier spring pressure or greater operator effort to effect return of the spool to a normal or neutral position. The jet flow of fluid through the opening in the face also prevents the fluid flow from exitting the throttling notches in the desired radial direction. Furthermore, the exit opening of the throttling notches in the periphery of the land acts as a second orifice in series with the opening in the face of the land creating an additional pressure drop and further upsets the fluid flow through the throttling notches.